1. Field of the Invention
Discontinuous tile distortions occur at tile boundaries of an image when the image is compressed or expanded on a tile-by-tile basis. The present invention relates to an image processing technique for reducing such tile distortions so that they become less noticeable.
2. Description of the Related Art
In such image compression techniques as JPEG2000, to reduce the capacity of a processing buffer, an image is compressed in units of a small section (tile). In such tile-by-tile image compression techniques, tile distortions occur at tile boundaries of an expanded image when the compression efficiency is increased excessively.
On the other hand, in such image compression techniques as JPEG, image compression is performed by using a very small block of about 8 pixels by 8 pixels as a unit of orthogonal transformation. It is known that block noise occurs even in such processing performed in units of a very small block.
Techniques disclosed in the following Patent documents 1 to 3 are known as techniques for reducing block noise of the above kind.
That is, Patent document 1 discloses a technique in which an average value of the pixels of a subject block is predicted by extrapolating pixel variations of adjacent blocks and pixel values of the subject block are level-shifted uniformly in accordance with the predicted value.
Patent documents 2 and 3 disclose techniques of reducing block noise by filtering block boundary portions.
[Patent document 1] U.S. Pat. No. 5,757,969 specification
[Patent document 2] U.S. Pat. No. 6,115,503 specification
[Patent document 3] U.S. Pat. No. 5,555,029 specification
The above-described Patent document 1 employs very small blocks of about 8 pixels by 8 pixels. It is relatively easy to predict an average value of the pixels of such a very small block based on surrounding very small blocks. Even if the prediction should prove wrong, the influence of the erroneous prediction is restricted in the very small block and hence the entire image does not have any noticeable failure.
On the other hand, the invention employs tiles each of which has as large an area as hundreds of pixels or more. Therefore, in the case of tiles, such variable factors as the pattern of each tile have great influence and hence the prediction operation of Patent document 1, for example, fails at a high probability.
Further, large-area tiles have long sidelines. Therefore, with the method of level-shifting pixel values of the entire tile uniformly, even if a tile distortion is removed at one location, new tile distortions may occur at other locations on the sideline.
On the other hand, where the filtering of Patent document 2 is applied to reduction of tile distortions, a problem arises that high-frequency components of an original pattern at a tile boundary may be lost.